Frank Sinatra Blue Moon "Happy New Year"

Blue Moon on New Year's Eve

For the first time in over a decade, New Year's Eve revelry, from early- evening snowshoe hikes to midnight fireworks, will take place by the light of a blue moon.

Savor the moment, astronomers say, because the phenomenon in which two full moons occur in the same month won't happen again in December until 2028.

"I encourage people to look up, enjoy the sight and think about all the interesting things that have happened" over the past decade, said Robert Stencel, professor of astronomy at the University of Denver, where public astronomy events are often held at the Chamberlin Observatory. " And be happy that we're still here to observe it."

I've heard the expression "once in a blue moon" all my life, and knew it was referring to "a long time", but I never thought about the term's origin. Today, my 90 year old father-in-law asked me if I knew what made a moon a "blue moon". I didn't know, and since we start the new year off in a "blue moon", I did some research. Interesting...very interesting.According to the popular definition, it is the second Full Moon to occur in a single calendar month.

The average interval between Full Moons is about 29.5 days, whilst the length of an average month is roughly 30.5 days. This makes it very unlikely that any given month will contain two Full Moons, though it does sometimes happen.

On average, there will be 41 months that have two Full Moons in every century, so you could say that once in a Blue Moon actually means once every two-and-a-half years.

Blue Moon on New Year's Eve

For the first time in almost twenty years, there's going to be a Blue Moon on New Year's Eve.Most months have only one full Moon. The 29.5-day cadence of the lunar cycle matches up almost perfectly with the 28- to 31-day length of calendar months. Indeed, the word "month" comes from "Moon." Occasionally, however, the one-to-one correspondence breaks down when two full Moons squeeze into a single month. Dec. 2009 is such a month. The first full Moon appeared on Dec. 2nd; the second, a "Blue Moon," will come on Dec. 31st.

This definition of Blue Moon is relatively new.

If you told a person in Shakespeare's day that something happens "once in a Blue Moon" they would attach no astronomical meaning to the statement. Blue moon simply meant rare or absurd, like making a date for the Twelfth of Never. "But meaning is a slippery substance," says Hiscock. "The phrase 'Blue Moon' has been around for more than 400 years, and during that time its meaning has shifted."

The modern definition sprang up in the 1940s. In those days, the Farmer's Almanac of Maine offered a definition of Blue Moon so convoluted that even professional astronomers struggled to understand it. It involved factors such as the ecclesiastical dates of Easter and Lent, and the timing of seasons according to the dynamical mean sun. Aiming to explain blue moons to the layman, Sky & Telescope published an article in 1946 entitled "Once in a Blue Moon." The author James Hugh Pruett cited the 1937 Maine almanac and opined that the "second [full moon] in a month, so I interpret it, is called Blue Moon."

That was not correct, but at least it could be understood. And thus the modern Blue Moon was born.

VIDEO: Frank Sinatra Blue Moon "Happy New Year"